1. Field of the Invention
The present invention relates to an organic electrolytic solution and a lithium secondary battery employing the same, and more particularly, to an organic electrolytic solution containing a fluorinated toluene compound and a lithium secondary battery having excellent lifetime and discharge capacity characteristics and improved high-temperature exposure characteristics obtained by employing the organic electrolytic solution.
2. Description of the Related Art
Accompanying the technological development of portable electronic devices which have become miniaturized and lightweight, high performance secondary batteries for supplying power to those portable electronic devices are in high demand, and research into lithium secondary batteries is being made intensively.
A lithium secondary battery is constructed by a cathode, an anode, an organic electrolytic solution for providing a movement path of lithium ions between the cathode and the anode, and a separator. Electrical energy is generated by an oxidation/reduction reaction when lithium ions are intercalated/deintercalated into/from the cathode and anode. The lithium secondary batteries are classified according to the kind of electrolyte used, into lithium ion batteries using liquid electrolyte and lithium ion polymer batteries. Since the lithium ion polymer batteries use solid electrolyte, they are free from the risk of leakage of an electrolytic solution, and have excellent processibility to be easily formed as a battery pack. Also, the lithium ion polymer batteries are lightweight and less bulky and have very small self-discharge rates. Due to such properties, the lithium ion polymer batteries are safe and can be easily fabricated as rectangular batteries and large-sized batteries, compared to the lithium ion batteries.
In the lithium secondary battery, a cathode and an anode are formed of materials capable of intercalating and deintercalating lithium ions. Materials for forming electrodes will now be described. First, as the cathode active material, lithium-containing metal oxides such as LiCoO2, LiMn2O4, LiNiO2 or LiMnO2, can be used. Among the lithium-containing metal oxides, a manganese group material such as LiMn2O4 or LiMnO2 has an undesirably small capacity, whereas it can be easily synthesized and is cheap and environmentally benign. Also, a cobalt group material such as LiCoO2 is expensive, while having excellent electric conductivity and voltage characteristics. In spite of the advantages of cost efficiency or high discharge capacity, a nickel group material such as LiNiO2 cannot be easily synthesized and must ensure the safety of a battery due to its high discharge capacity.
As the anode active material, metallic lithium, lithium alloys or carbon materials can be used. However, metallic lithium has a problem of short circuiting caused by formation of dendrites, which entails a danger of explosion of a battery. Thus, recently, carbon materials have been more favorably used as the anode active materials.
Since the properties of a lithium secondary battery are due to complex reactions between a cathode and an electrolytic solution or between an anode and an electrolytic solution, use of an appropriate organic electrolytic solution is one of important factors for improving the performance of the lithium secondary battery. The organic electrolytic solution is an ion-conductor prepared by dissolving lithium salts in an organic solvent and should be excellent in view of conductivity of lithium ion and chemical and electrochemical stability with respect to electrodes. Also, the organic electrolytic solution must be cheap and usable over a broader range of the working temperature. Thus, an organic solvent having high ion conductivity and dielectric constant and low viscosity, is preferably used as the organic solvent consisting of the organic electrolytic solution.
However, there is no single organic solvent which can satisfy the above-described requirements in practice. Thus, the composition of an organic solvent contained in the organic electrolytic solution may include a two-component system comprising a high dielectric constant solvent and a low viscosity solvent (see U.S. Pat. No. 5,437,945), or a three-component system further comprising a third organic solvent having a low boiling point in addition thereto (see U.S. Pat. No. 5,474,862). Use of such mixed organic solvents increases the mobility of lithium ions, leading to improvement of ion conductivity, and enhances the initial discharge capacity of a battery.
It has been known that the lifetime and capacity characteristics of a battery are importantly influenced by the surface reactivity between the carbon material used as the anode active material and the electrolytic solution, in the lithium secondary battery, in particular, the lithium ion battery. Thus, reaction of the anode active material and the electrolytic solution, rather than reaction between the cathode active material and the electrolytic solution, is preferably taken into prior consideration when the composition of an electrolytic solution is selected.
Recently, ethylene carbonate that seldom reacts with the anode active material is more favorably used than propylene carbonate that has reactivity with the anode active material.
The high-temperature exposure characteristic of a battery has become adopted as a new item for evaluating the performance of the battery. The high-temperature exposure characteristic is important because a battery exposed to a high-temperature for a long time cannot be used any longer due to an increase in the internal pressure, resulting in opening of a vent.
To solve the above problems, it is a first object of the present invention to provide an organic electrolytic solution having improved high-temperature exposure characteristic.
It is a second object of a lithium secondary battery having improved lifetime and discharge capacity characteristics by employing the organic electrolytic solution.
To achieve the first object, there is provided an organic electrolytic solution containing an organic solvent and a lithium salt, wherein the organic solvent comprises 20 to 60% by volume of ethylene carbonate, 20 to 70% by volume of dialkyl carbonate and 5 to 30% by volume of a fluorinated toluene compound.
To achieve the second object, there is provided a lithium secondary battery including a cathode having a lithium-containing metal oxide, an anode having metallic lithium, lithium alloy or a carbon material, and an organic electrolytic solution having a lithium salt and an organic solvent containing 20 to 60% by volume of ethylene carbonate, 20 to 70% by volume of dialkyl carbonate and 5 to 30% by volume of a fluorinated toluene compound.